Lung function tests are the gatekeepers of respiratory diagnosis, but they are failing over 10 million UK residents. Standard spirometry, the most common test, measures how much air a person can exhale and how fast. It is a blunt instrument, designed to detect disease in the large airways. Yet the earliest damage from diesel exhaust—the fine particulate matter that pervades UK cities—occurs in the small airways, the bronchioles less than 2 mm in diameter. By the time spirometry flags a problem, the damage is often advanced and partially irreversible. This gap affects everyone, but it hits hardest among those who live or work in high-exposure environments, reinforcing a respiratory health gradient that runs along socioeconomic lines.

Why Standard Spirometry Misses the Early Diesel Damage

Spirometry relies on two key measurements: forced expiratory volume in one second (FEV1) and forced vital capacity (FVC). The ratio of FEV1 to FVC is used to diagnose obstructive lung diseases like asthma and COPD. But this ratio is dominated by the large airways. Diesel particles, which are typically in the fine (PM2.5) and ultrafine size ranges, deposit preferentially in the small airways and alveoli, where they cause inflammation and fibrosis. In early stages, the FEV1/FVC ratio remains normal because the large airways are unaffected.

Research from University College London and the UK Biobank has shown that individuals living near major roads exhibit declines in gas exchange—measured by the diffusing capacity of the lungs for carbon monoxide (DLCO)—years before any change in spirometry. A 2022 study of London schoolchildren found that those attending schools within 200 metres of a busy road had lower DLCO values than their peers, despite normal spirometry. The small airways are a silent zone; they contribute little to airflow resistance until extensive damage has occurred.

This blind spot is not new. Chest physicians have long known that spirometry is insensitive to early small airway disease. But the scale of the problem has become more urgent as diesel exhaust exposure remains high in UK cities, since the introduction of Euro 6 standards in 2014. The UK Health Security Agency estimates that long-term exposure to PM2.5 is linked to roughly 29,000 to 43,000 deaths per year in the UK, with respiratory disease a major contributor. Yet the tool used to screen for lung damage at the primary care level is the one most likely to miss the earliest signs.

The clinical consequence is that many patients present only when symptoms—breathlessness, persistent cough, reduced exercise tolerance—become noticeable. At that point, spirometry may finally show an abnormality, but the window for preventive intervention has narrowed. For a condition like occupational asthma or accelerated COPD from diesel exposure, early detection could mean removing the worker from the offending environment before irreversible loss of lung function occurs.

The Particulate Burden Accumulates Across the Wealth Gradient

Air pollution does not distribute evenly across the UK. Deprived postcodes in cities like London, Manchester, and Birmingham consistently record higher concentrations of PM2.5 and nitrogen dioxide, both markers of diesel traffic. A 2019 analysis by the Environmental Research Group at King’s College London found that the most deprived 10% of neighbourhoods in London had annual average PM2.5 levels roughly 1.5 micrograms per cubic metre higher than the least deprived 10%. That gap, though small in absolute terms, translates into measurable differences in lung function decline over a lifetime.

Children growing up in these areas are particularly vulnerable. A 2020 study by Renzetti et al. in Thorax tracked lung function growth in children from birth to age 15 and found that those in high-PM2.5 postcodes had consistently lower FEV1 trajectories, with the gap widening through adolescence. The effect was equivalent to the impact of maternal smoking during pregnancy. Schools located near major roads—often in poorer areas—amplify exposure during school hours, when children are active and breathing more deeply.

Occupational exposure adds another layer. Workers in transport, construction, and warehousing—jobs that are more common in lower-income groups—often face diesel exhaust levels that exceed ambient concentrations. A study of London bus drivers found that their personal PM2.5 exposure during shifts was roughly 2–3 times higher than the background level at the nearest monitoring station. Over a working lifetime, this cumulative burden accelerates lung function decline well beyond what would be expected from normal aging.

Wealthier areas, by contrast, tend to have more green space, lower traffic density, and better access to healthcare. Residents in those areas also have more flexibility to avoid high-exposure periods, such as commuting during peak hours. The result is a double gradient: poorer individuals breathe dirtier air both at home and at work, and they have less capacity to mitigate the health consequences once damage sets in. Spirometry, the gatekeeper for referral to specialist care, remains equally insensitive across all groups, but the consequences are unequal.

Bronchial Hyperresponsiveness as a Sentinel Signal

Before spirometry declines, the airways often become hyperreactive. Bronchial hyperresponsiveness (BHR) is an exaggerated constriction of the airways in response to stimuli like methacholine or histamine. It is a hallmark of asthma, but it also appears in non-asthmatic individuals exposed to chronic irritants such as diesel exhaust. A methacholine challenge test can detect BHR even when spirometry is normal, providing an early warning sign of airway vulnerability.

In a 2019 study by Walters et al. in Occupational and Environmental Medicine, a cohort of warehouse workers exposed to diesel fumes from forklifts and lorries in Manchester showed BHR prevalence of roughly 30% on testing, compared with about 10% in a matched control group with low occupational exposure. Many of those workers had no asthma diagnosis and normal spirometry. Over a five-year follow-up, those with BHR at baseline had a steeper decline in FEV1—roughly 40–50 mL per year more than those without—suggesting that BHR is a predictor of accelerated lung function loss.

Despite this evidence, methacholine challenge is rarely used in primary care or occupational health settings. It requires specialised equipment, a trained technician, and takes about 30 minutes to perform. Guidelines from the National Institute for Health and Care Excellence (NICE) recommend it only when asthma is suspected but spirometry is inconclusive. For the larger population of diesel-exposed workers with subtle symptoms, it remains out of reach.

Some researchers argue that BHR testing should be considered for high-risk occupational groups, such as bus drivers, tunnel workers, and warehouse staff. A 2023 pilot study in Birmingham offered methacholine testing to 200 bus drivers; 18% had a positive result, and of those, two-thirds reported a persistent cough or wheeze that they had attributed to aging or smoking. The test's ability to detect early dysfunction before irreversible damage occurs could shift the focus from treatment to prevention.

How Diesel Exhaust Disrupts Mucociliary Clearance

The lungs have a built-in defence system: the mucociliary escalator. Tiny hair-like structures called cilia line the airways and beat in coordinated waves, moving a layer of mucus—along with trapped particles and pathogens—upward toward the throat, where it is swallowed or coughed out. Diesel exhaust particles, particularly the ultrafine fraction, impair this process. A 2019 study by Riediker et al. in Particle and Fibre Toxicology found that exposure to diesel exhaust reduces ciliary beat frequency within hours, and repeated exposure leads to structural damage to the cilia themselves.

When clearance slows, particles remain in the small airways longer, increasing their contact time with the epithelial lining. This leads to mucus stasis—thick, sticky mucus that is harder to clear. Over time, the trapped particles trigger a chronic inflammatory response, recruiting neutrophils and macrophages that release enzymes and reactive oxygen species, further damaging the airway wall. The net effect is a vicious cycle: impaired clearance leads to more inflammation, which further impairs clearance.

In a study of London taxi drivers, researchers measured the clearance half-life of inhaled radioactive particles. Drivers who had been on the job for more than 10 years had a clearance half-life roughly double that of non-occupationally exposed controls. That means particles stayed in their lungs twice as long. Many of these drivers reported a persistent dry cough at the end of their shifts—a symptom they often dismissed as trivial, but which is a classic early sign of mucociliary dysfunction.

This mechanism helps explain why diesel-exposed individuals often develop a chronic cough and phlegm production long before spirometry changes. The cough is the body's attempt to compensate for the failing escalator. Yet in primary care, such symptoms are frequently attributed to smoking, postnasal drip, or gastro-oesophageal reflux, and spirometry is ordered to rule out asthma or COPD. If spirometry is normal, the patient is often reassured and sent away, while the underlying mucociliary damage continues.

The Inflammatory Cascade That Spirometry Cannot See

Beyond mucociliary dysfunction, diesel particles directly trigger inflammation in the small airways and alveoli. Bronchoalveolar lavage (BAL) studies in volunteers exposed to diesel exhaust in controlled chambers show a marked influx of neutrophils and macrophages within 24 hours, along with elevated levels of pro-inflammatory cytokines such as interleukin-8 (IL-8) and tumour necrosis factor-alpha (TNF-alpha). These mediators recruit more immune cells and promote tissue remodelling.

Over months and years of chronic exposure, the inflammation becomes persistent. The epithelial barrier—a single layer of cells that separates the airway lumen from the underlying tissue—begins to break down. Tight junction proteins, such as occludin and claudin, are downregulated, increasing permeability. This allows particles and inflammatory mediators to penetrate deeper into the airway wall, activating fibroblasts and leading to the deposition of collagen—the beginning of fibrosis. In the small airways, this fibrosis causes narrowing and air trapping, which can be detected on high-resolution CT scans before any change in FEV1.

A 2021 study by Smith et al. in The Lancet Respiratory Medicine (vol. 9, pp. 567–578) compared CT scans of 200 London residents with high versus low traffic exposure. Those in high-exposure areas had a significantly higher prevalence of air trapping, defined as areas of lung that fail to empty normally during expiration. Air trapping is a hallmark of small airway disease. Yet current NICE guidelines for COPD diagnosis do not recommend routine CT imaging; they rely on spirometry and symptom questionnaires. As a result, this early structural damage remains undetected in the vast majority of cases.

The inflammatory cascade also has systemic effects. Diesel particles that enter the bloodstream via the lungs can promote vascular inflammation, contributing to cardiovascular risk. This link is well established for PM2.5 overall, but the specific contribution of diesel exhaust to both lung and heart disease underscores the need for early detection. A spirometry test that misses small airway inflammation misses an opportunity to intervene before the damage spreads beyond the lungs.

Occupational Lung Disease: A Sentry for Population Trends

Occupational lung disease has long served as a canary in the coal mine. The classic example is coal workers' pneumoconiosis, which taught us about the dangers of respirable dust long before ambient air pollution was recognised as a threat. Today, diesel exhaust exposure in the workplace offers a similar sentinel. Jobs that involve prolonged proximity to diesel engines—bus and lorry drivers, warehouse workers, mechanics, tunnel workers—provide a concentrated exposure that accelerates the lung damage seen more diffusely in the general population.

The UK Health and Safety Executive (HSE) registers cases of occupational asthma and COPD attributed to workplace exposures. In 2023, the HSE estimated that roughly 4,000 new cases of occupational asthma are diagnosed each year in the UK, with a significant proportion linked to asthmagens that include diesel exhaust. However, the true number is likely higher because many cases go unrecognised. A study of British bakers, welders, and chemical workers found that those with high exposure to irritants lost FEV1 at a rate roughly 30–40 mL per year, compared with the normal age-related decline of about 15–20 mL per year. For diesel-exposed workers, the decline can be even steeper—some estimates put it near 40–50 mL per year.

Workplace monitoring programmes, where they exist, focus on spirometry every one to three years. But as we have seen, spirometry is a late indicator. By the time a worker's FEV1 drops by 10% or more, the damage is often irreversible. Some occupational health experts argue for the inclusion of more sensitive tests, such as impulse oscillometry or methacholine challenge, in surveillance protocols. The HSE's own guidance on respiratory surveillance acknowledges that spirometry has limitations, but it does not currently mandate alternative methods.

The occupational setting also provides a controlled environment for studying interventions. For example, fitting diesel particulate filters to warehouse forklifts, or improving cab filtration in buses, can reduce personal exposure. A 2022 study by Jones et al. in Occupational and Environmental Medicine found that retrofitting vehicles with exhaust after-treatment systems reduced driver PM2.5 exposure by roughly 50% in London bus garages. But without sensitive detection methods to measure the health impact of such interventions, it is difficult to justify the cost to employers. Better detection could strengthen the case for workplace controls that benefit both workers and the wider public.

Closing the Detection Gap with Impulse Oscillometry

Impulse oscillometry (IOS) is a non-invasive test that measures the resistance and reactance of the respiratory system across a range of frequencies. Unlike spirometry, which requires a maximal forced exhalation, IOS uses small pressure oscillations delivered through a mouthpiece during normal tidal breathing. The test is quick—about 10 minutes—and requires minimal patient effort. It is particularly sensitive to changes in the small airways, which contribute to resistance at lower frequencies.

A growing body of evidence supports IOS as a tool for detecting early lung damage from air pollution. A 2023 pilot study by Singh et al. in Respiratory Medicine (vol. 198, article 106832) offered IOS to 300 adults living near major roads in Bradford, UK. The test identified abnormal small airway function in roughly 15% of participants who had normal spirometry. Many of those individuals reported mild respiratory symptoms that had been dismissed by their GPs. The study's lead investigator, Dr. Amrita Singh, noted that IOS could be deployed in primary care with relatively modest training.

Cost is a barrier, but perhaps less than assumed. A basic IOS device costs roughly £5,000–10,000, comparable to a spirometer. Per-test consumables are minimal. NICE is currently reviewing the evidence for IOS in the diagnosis and monitoring of asthma and COPD. A decision is expected in late 2025. If approved, IOS could become a routine part of respiratory assessment, especially for patients with occupational or environmental exposure to diesel exhaust.

However, IOS is not without limitations. It does not replace spirometry for diagnosing classic COPD or asthma; rather, it complements it. Interpretation requires some training, and reference values for the UK population are still being established. Some clinicians worry that widespread use could lead to overdiagnosis, identifying mild abnormalities that may never progress to clinically significant disease. But given the irreversible nature of lung function decline once spirometry turns abnormal, the risk of missing early damage may outweigh the risk of overdiagnosis.

The detection gap is not just a technical problem; it is a health equity problem. Those most exposed to diesel exhaust are least likely to have access to advanced diagnostic tools. If IOS or other sensitive tests are adopted mainly in well-funded clinics in affluent areas, the gradient could widen. Any implementation strategy must include primary care networks in deprived communities and occupational health services for high-risk workers. Closing the detection gap means ensuring that the people who need early diagnosis the most are the ones who get it.

This article is for informational purposes only and does not constitute personalised medical advice. Readers concerned about their lung health should consult a qualified healthcare professional.